Tubular materials

ABSTRACT

Tubular workpieces of oriented polymers such as polyethylene polyester and PVC are formed by drawing a workpiece over an expanding former in the absence of any force perpendicular to the axis of the workpiece. The processes do not require the use of any liquid lubricant. They enable greater degrees of expansion and hence orientation in the direction perpendicular to the axis to be achieved and result in products of improved appearance and strength.

This is a Rule 60 Continuation of application Ser. No. 08/013,164, filed2 Feb. 1993, now abandoned, which is a Rule 62 Continuation ofapplication Ser. No. 07/887,392, filed 21 May 1992, now abandoned, whichis a Rule 62 Continuation of application Ser. No. 07/767,365, filed 30Sep. 1991, now abandoned, which is a Rule 62 Continuation of applicationSer. No. 07/439,888, filed 20 Nov. 1989, now abandoned.

This invention relates to processes for the production of tubularmaterials of an orientable thermoplastic polymer by solid phasedeformation and to the products of those processes.

BACKGROUND OF THE INVENTION

Previous attempts to introduce biaxial orientation into tubularmaterials formed from orientable thermoplastic polymeric materials bydrawing them over an expanding former have involved only a limiteddegree of expansion in the hoop direction i.e. in the directionperpendicular to the axis of the tube. British Patent 1456222 describesa process and apparatus used to draw a tube of thermoplastic polymerover an expanding former in order to calibrate the internal dimensionsof the tube. The processes described require the application of a liquidlubricant to the inner surface of the workpiece prior to its passageover the former. In our British Patent 2156733 we have describedprocesses in which the tubular material is drawn simultaneously througha die and over an expanding former.

DESCRIPTION OF THE INVENTION

We have now discovered that uniform biaxially oriented products can beproduced by processes which comprise drawing the tubular material overan expanding former without the need to apply a liquid lubricant. Suchprocedures are advantageous in that the products may have a greaterdegree of orientation in the hoop direction and correspondinglyuniformly greater strength in that direction than has previously beenattainable. Furthermore insofar as they involve only drawing the tubularmaterial over a former they offer advantages in simplicity of operationand in the improved appearance of the product.

Accordingly from one aspect our invention provides a process for theproduction of a biaxially oriented tubular material which comprisesdrawing a hollow workpiece comprising an orientable thermoplasticpolymer over an internally positioned expanding formed without theapplication of a liquid lubricant to the interior of the workpiece whichis characterised in that the deformation of the workpiece is carried outin the absence of any external force acting in a direction which isperpendicular to the axis of the workpiece.

The term "hollow workpiece" as used herein includes tubes and othercross-sections of hollow stock. The term includes both billets and otherforms of stock of greater length. Continuous stock, which may be formedas the process is performed, may be utilised. Examples includeopen-ended elongate workpieces of substantially constant cross-section,desirably with an axis of symmetry; for example, hollow workpieces ofcircular, elliptical, square, rectangular or triangular cross-section.

The orientable thermoplastic polymers may be a semicrystalline polymersuch as polyethylene, polypropylene or polyvinylidene fluoride, anamorphous crystallising polymer such as polymethylmethacrylate or acrystallisable polymer such as polyvinylchloride, polyesters orpolycarbonates. Examples of preferred classes of such polymers areunsubstituted or mono- or poly- halo-, for example chloro- or fluoro-substituted vinyl polymers, unsubstituted or hydroxy-substitutedpolyesters, polyamides, polyetherketones and polyacetals. Specifically,linear homo- or copolymers of ethylene or propylene with at least onecomonomer; a vinyl chloride polymer; a vinyl fluoride polymer or avinylidene fluoride polymer; PHB; PEEK; or a homo- orco-polyoxymethylene may be utilised.

The hollow workpiece is preferably essentially unoriented beforedeformation. The term "essentially unoriented" as used herein means thatthe hollow workpiece has incurred no orientation other than that minoramount which might be induced during formation of the workpiece,(including orientation produced in the semi-molten state) for exampleduring billet moulding or melt extrusion, or during any subsequentshaping thereof, for example by machining, prior to its passage over theformer. However, workpieces which have been oriented to a considerabledegree may be advantageously utilised in the processes of the presentinvention. For example workpieces which have been produced by extrusionthrough a die or by deformation in the solid phase through a die may beutilised.

Where it is not possible to use the polymer as received as a hollowworkpiece, the workpiece may be formed either in the batch orcontinuously. In either case care should be taken to ensure that theworkpiece cools at an appropriate rate so that the resulting hollowworkpiece will be without defects. The techniques for forming suchworkpieces are well known in the art. For example in the case of ahollow workpiece formed from polyethylene in a batch process as abillet, the following general methods have been found to be suitable.The polymer may be melted in a screw extruder; extruded or injected intoa billet mould at a temperature about 30° C. above its melting point andcooled under elevated pressure for 5 hours, or it may be melted in anextruder; extruded or injected into a cold billet mould; transferred toan oven for 4 hours under ambient pressure at a temperature below itsmelting point but above its crystallisation temperature; and thereafterallowed to cool in the oven after the heating has been switched off. Thepolymer may also be injection moulded into an air or water cooled billetmould. Progressive immersion in the cooling fluid is preferred, therebyensuring that solidification of the polymer takes place from the bottomof the mould thereby preventing the formation of internal voids due toshrinkage. A hollow workpiece may be formed in the batch by including amandrel of appropriate cross-section, usually coaxially, in the mould.Alternatively, stock of circular or other hollow cross-section may beproduced continuously using one of several processes known to thoseskilled in the art.

For any particular polymer a steady state process may be obtained byadjusting the parameters of the process. The actual values will dependon the nature of the polymer and the dimensions of the polymer beforeand after deformation. In particular we have discovered that the minimumhoop draw ratio for any particular polymer should be sufficiently highso as to ensure that a uniform product is produced. Preferably the axialdraw ratio is also maintained at a level which ensures this. Where anon-uniform product is produced under particular conditions these ratioscan and preferably are increased. The actual values required to producea uniform product of a particular polymer may readily be determined byroutine experiment.

The hoop draw ratio is defined as the ratio of the final hoop dimensionto the initial hoop dimension and the axial draw ratio is the ratio ofthe initial bulk cross-sectional area of the hollow workpiece to thefinal bulk cross-sectional area of the product. The hoop draw ratio isconventionally expressed as the ratio of the maximum dimension of theproduct to maximum dimension of the workpiece. Because the walls of atubular workpiece have a finite thickness the hoop draw ratio can beexpressed either as the inner hoop draw ratio (i.e. the ratio of theinside diameter of the product to the inside diameter of the workpiece)or the outer hoop draw ratio (i.e. the ratio of the outside diameter ofthe product to the outside diameter of the workpiece). The inner hoopdraw ratio will always be the larger.

For polyolefins and in particular linear homo and copolymericpolyethylenes the preferred inner hoop draw ratio is at least 1.2 andmore preferably at least 1.5 and most preferably at least 2.0. Thepreferred axial draw ratio is at least 2 and preferably greater than 3.The outer hoop draw ratio may be less than 1 but is preferably at least1 and more preferably at least 1.5 or 2.0. The ratio of the axial drawratio to the inner hoop draw ratio is preferably at least 1 and lessthan 4, most preferably less than 2. For polypropylene it may bepreferable to utilize large axial draw ratios even up to 7 or 8 and theratio of the axial draw ratio to the hoop draw ratio is correspondinglyincreased.

For these polymers the hollow workpiece is desirably heated to aprocessing temperature within 60° C. of the melting point of the polymerprior to deformation. More particularly for polyethylene polymers havinga weight average molecular weight of from 50,000 to 150,000 thetemperature is preferably from 70° C. to 100° C. and for polymers havinga weight average molecular weight of above 300,000 from 70° to 120° C.For linear homo- and copolymers of polypropylene of weight averagemolecular weight from 150,000 to 800,000 the hollow workplace isdesirably heated to a temperature from 20° C. to 170° C., preferably 90°C. to 130° C. The processing temperature is only a nominal temperaturesince the process is not isothermal but should be within the range whichthe polymer is amenable to deformation.

The processes of the invention find particular application in theproduction of tubular polyolefin materials wherein the thickness of thewalls of the product tube is from 0.1 to 5.0 mm.

By "bulk cross sectional area" is meant the area of the polymericmaterial substantially normal to the machine direction. Thus for tubularworkplace having an exterior diameter D₂ and an interior diameter D₁ thebulk cross sectional area is ##EQU1##

A preferred class of polyesters is those which are derivable from thereaction of at least one polyhydric alcohol, suitably a linearpolyhydric alcohol, preferably a diol such as a linear C₂ to C₆ diol,with at least one polybasic acid, suitably a polycarboxylic acid. Thealcohol is preferably an alicyclic or aliphatic such alcohol; forexample cyclohexane-dimethanol or a linear C₂ to C₆ alkylene diol suchas ethylene glycol, 1,3-propylene glycol, or 1,4-butylene glycol,especially ethylene glycol. The acid is preferably an aromatic,alicyclic or aliphatic such acid; for example a mono- or polycarbocyclic aromatic acid such as an aromatic dicarboxylic acid e.g. o-,m- or p-terephthalic acid; 2,6- or 1,5-naphthalene dicarboxylic acid or1,2 dihydroxybenzoic acid especially terephthalic acid. Examples ofsuitable polyesters include polyethylene 2,6-naphthalate, polyethylene1,5-naphthalate, polytetramethylene 1,2-dihydroxybenzoate, polyethyleneterephthalate, polybutylene terephthalate and copolyesters, especiallyof ethylene terephthalate.

With polyesters the inner hoop draw ratio achieved is preferably atleast two and preferably at least 3. At the same time the preferredaxial draw ratio is at least two and preferably at least three. Theprocessing temperature is preferably from 55° to 110° C. or even 55° to120° C.

The processes of the present invention find particular application inthe production of tubular materials comprising polyesters wherein thethickness of the wall of the product tube is from 0.2 to 0.6 mm. Suchtubes are preferably produced from tubular billets utilising adeformation ratio of from 2 to 4.

For homo- or copolyoxymethylene a draw temperature of 80° C. to 170° C.,preferably 150° C. to 170° C. is suitable and for vinylidene fluoridepolymers a draw temperature of from 80° C. to 165° C. is suitable.

For vinyl chloride polymers especially polyvinyl chloride itself thepreferred axial and hoop draw ratios are at least 1.2 preferably atleast 1.5 or 2.0. Values of not greater than three may also bepreferred. The ratio of the axial draw ratio to the inner hoop drawratio may be less than unity and is preferably in the range 0.5 to 1.0.

The processing temperature may be further controlled by utilising aheated former and/or a temperature controlled chamber which extendsdownstream. Certain polymers may also be heated by subjecting them to adielectric field, as disclosed in EPC 0084274 and U.S. Pat. No.3,364,294.

It is feasible to use draw speeds greater than 200 cm min⁻¹ in thedrawing process of this invention. Speeds of 50 cm min⁻¹ or more arepreferred. Lower draw speeds may be used if desired.

The process may be operated by drawing the workpiece directly over thesurface of the former. However, it may be preferred to lubricate thesurface of the former with a non-liquid lubricant as this can improvethe quality of the interior surface of the workpiece and also serve toreduce the force required to draw the workpiece. Conveniently thesurface is lubricated by use of air (which is preferably heated to anappropriate temperature). These advantages may also be achieved byvarying the nature of the former, e.g. by utilising a former whosesurface is formed by a multiplicity of small rollers or sphericalelements.

The improved surface properties are of particular relevance to processeswhich utilise workpieces formed from transparent polymers. The processesof the present invention find particular application to the productionof transparent products since the exterior surface of the workpiece neednot come into contact with the interior surface of a die as is the casein the process of our UK Patent 2156733. Processes for the production oftransparent workpieces in which the exterior surface of the hollowworkpiece does not contact another solid surface form a preferred aspectof the present invention. The use of a lubricated surface on the formerrepresents a preferred aspect of this embodiment.

In some circumstances it may be preferable to employ a die having aninternal diameter equal to the external diameter of the workpiece to actas a guide member to support the workpiece prior to its being drawn overthe former. Contact with the surface of such a guide may detract fromthe surface properties of a transparent workpiece and is thereby lesspreferred. However, when utilising opaque workpieces or transparentworkpieces intended for use in non-decorative applications, the use ofsuch a guide may well be convenient.

In performance of the invention a nose formed on the hollow workpiece isadvanced to protrude beyond the former and is secured to tensioningmeans applied from the exit side thereof. A suitable arrangementincludes a hauloff comprising a pair of serrated jaws in which the noseis gripped; a high tensile cable one end of which cable is attached tothe jaws, the other to a winch or a loading station to which a turningmoment or mass may be applied thereby applying a draw tension to thenose. The hauloff may also comprise, instead of a cable, any tensiontransmitting means used in the metal drawing art including a chain, arack and pinion mechanism, a screw mechanism and a hydraulicallyoperated draw mechanism. The hauloff may further comprise a pair ofcontinuous contra-rotating friction belts, generally known as a"caterpillar" ("CATERPILLAR" is a registered Trademark).

The draw tension should be sufficient to draw the hollow workpiece overthe former but insufficient to cause tensile failure of the article;that is, the draw tension should be such that the true stress at anypoint of the product does not exceed its fracture stress at that point.A suitable maximum value of draw tension may readily be determined byroutine experiment.

After a grippable length of the hollow workpiece has been drawn over theformer any unsuitably oriented part of its nose may be removed and theoriented grippable length re-gripped thereby enabling a higher initialload to be applied.

A batch process may be converted to a semi-continuous one by putting theupstream end of the deforming hollow workpiece and the downstream end ofstock of the same cross-sectional both in contact with a hot, stainlesssteel plate; removing the plate and welding the two polymer surfaces.Preferably such a weld should be at an angle of 45° C. or less to theaxis of the stock.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described by way of example by reference tothe accompanying FIG. 1 which represents a schematic side elevation ofthe apparatus diametrically sectional along the machine direction.

In the drawing the apparatus consists of a former 1 and upstream thereofan oven 2. The former 1 is supported by rod 3. Hauloff jaws 4 arepositioned downstream from the former 1 and are connected to the winch(not shown). The former has an annular slit 6 connected to pipe 7.

In use the initial hollow workpiece 5 which has been machined at one endto provide a nose is inserted over the former 1 and rod 3. The nose isgripped in the jaws 4 and load applied slowly at first so that theplastic strain is increased without causing tensile failure. After thisstart-up a steady drawing speed at a steady drawing load is established.

EXAMPLES

The invention is illustrated by the following Examples:

Example 1

The workpiece comprised a tube of isotropic polypropylene (ICI grade GSE108) having an internal diameter of 18.5 mm and an external diameter of26.5 mm.

As a preliminary operation a belled end was formed on one end of thetube by heating that end to a temperature of 140° C. and inserting a hotmetal plug in the shape of a taper nosed cylinder into the bore of thetube. The end of the tube and the plug were immersed in hot oil untilthe end had deformed sufficiently.

The tube was then mounted in the apparatus illustrated in FIG. 1. Thetube was drawn over a former having a maximum diameter of 70 mm and acartridge heater through which additional heat could be applied. Thetube was drawn at a speed of 13 cm min⁻¹ at a draw temperature of 135°C. and using a draw force of 1.55 kN.

The drawn tube was substantially uniform throughout its length having anexterior diameter of 62 mm and a wall thickness of 0.225 mm. The hoopdraw ratio of the outer surface was 2.25. The axial draw ratio was 6.5.

The 10 sec creep modulus at 0.1% strain of samples cut from the drawntube was determined for the axial and hoop direction using the standarddead loading creep method described by Gupta and Ward (V. B. Gupta andI. M. Ward--J. Macromol. Sci. B1 373 1967). The 10 sec creep modulus inthe axial direction was 3.7 GPa and in the hoop direction 1.7 GPa.

Example 2

A tubular billet having an external diameter of 25 mm and an internaldiameter of 17 mm of a clear amorphous copolyester (Eastmann 9921) wasfirst deformed so as to provide a belled nose. The tube was placed in anoven at a temperature of 110° C. in such a way that a zone of a lengthof approximately 15 centimeters was heated. After 5 minutes the tube wasremoved and quickly inflated with room temperature compressed air to apressure of 620 KN/m² whereupon a bubble is blown in the heated part.After depressurising and cooling one end of the bubble is removed so asto produce a belled end on the tube.

The tube was then mounted in the apparatus illustrated in FIG. 1 Airheated to a temperature of 95° C. was passed through the slit 6 via thepipe 7 so as to float the tube above the surface of the former. The airpressure used was 275 KN/m² and the air flow was 40 cubic feet per hour.

The oven was maintained at a temperature of 90° C. throughout thedrawing process. The tube was drawn at a speed of 20 cm/min using a drawforce of 1.5 KN.

The drawn tube was transparent and had an exterior diameter of 76 mm.The wall thickness was 0.31 mm. The axial draw ratio was 3.5. The hoopdraw ratio of the outer surface was 3.0.

The Youngs modulus of samples of the drawn tube having a width of 9.8 mmand a gauge length of 10.6 cms were determined in an Instron tensiletesting machine at a strain rate of 3.3×10⁻⁴ sec,⁻¹.

The stress strain curve was linear up to at least 0.5% strain. Theinitial modulus for the axial and hoop directions are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Direction Initial Modulus (GPa)                                                                       Extension to Break (%)                                ______________________________________                                        Axial     3.1           63                                                    Hoop      3.0           68                                                    ______________________________________                                    

Example 3

A series of billets were formed and drawn using the apparatus as shownin FIG. 1. The conditions employed and the results achieved aresummarised in Table 2. For ease of comparison the details of Examples 1and 2 are included in this table.

                                      TABLE 2                                     __________________________________________________________________________    Examples of Die free drawing                                                         Billet size                                                                         Mandrel                                                                           Draw                                                                              Draw Draw                                                                             Axial                                                                            Inner surface                                                                       Outer surface                                  ID OD size                                                                              Temp.                                                                             Speed                                                                              Force                                                                            draw                                                                             Hoop draw                                                                           Hoop draw                               Material                                                                             (mm)                                                                             (mm)                                                                             (mm)                                                                              (°C.)                                                                      (cm/min)                                                                           (KN)                                                                             ratio                                                                            ratio ratio                                   __________________________________________________________________________    Polypropylene                                                                        18.5                                                                             26.5                                                                             70.0                                                                              135 13.0 1.55                                                                             6.5                                                                              3.33  2.25   Ex 1                             GSE 108                                                                              "  "  "   155 6.5  1.10                                                                             6.15                                                                             3.61  2.5                                            "  "  "   "   65.0 1.10                                                                             8.60                                                                             2.65  1.85                                    PET    17.0                                                                             25.0                                                                             80.0                                                                               90 20.0 1.5                                                                              3.5                                                                              4.4   3.0    Ex 2                             (Eastman 9221)                                                                HDPE   21.0                                                                             62.0                                                                             62.0                                                                              115 3.0  8.0                                                                              3.8                                                                              2.7   1.03                                    00-240 "  "  "   "   50.0 8.0                                                                              5.9                                                                              2.6   0.97                                    PVC    32.0                                                                             42.0                                                                             70.0                                                                              100 2.0  2.65                                                                             1.47                                                                             1.90  1.55                                    (BS 3505 CL7)                                                                        "  "  "   "   12.0 2.65                                                                             1.55                                                                             1.80  1.47                                    __________________________________________________________________________

What we claim is:
 1. A process for the production of a biaxiallyoriented tubular material having a wall thickness of at least 0.215 mmwhich comprises the steps of:providing an expanding former having anexterior surface; deforming a hollow workpiece of wall thickness atleast 4 mm having an interior surface and an exterior surface andcomprising an orientable thermoplastic polymer, by drawing it withapplication of heat directly over said exterior surface of saidexpanding former, in the solid phase, without the application of anyliquid or lubricant to said interior surface of the workpiece duringdeformation thereof; and drawing said deformed workpiece off saidexpanding former using a tension transmitting mechanism exerting a drawtension sufficient to draw the workpiece over the former butinsufficient to cause tensile failure of the workpiece to produce drawnbiaxially oriented material in tubular form having a wall thickness ofat least 0.215 mm; the process being carried out without the workpiececontacting an external solid surface at the deformation, such that thereis no external force acting on the workpiece in a directionperpendicular to the axis of the workpiece at the deformation.
 2. Aprocess according to claim 1, wherein the thermoplastic polymer is asemi-crystalline polymer.
 3. A process according to claim 2, wherein thethermoplastic polymer is a polyolefin.
 4. A process according to claim3, wherein the thermoplastic polymer is a polyethylene.
 5. A processaccording to claim 3, wherein the thermoplastic polymer is apolypropylene.
 6. A process according to claim 2, wherein the inner hoopdraw ratio is at least 1.2.
 7. A process according to claim 2, whereinthe inner hoop draw ratio is at least 1.5.
 8. A process according toclaim 2, wherein the axial draw ratio is at least
 2. 9. A processaccording to claim 8, wherein the axial draw ratio is at least
 3. 10. Aprocess according to claim 5, wherein the axial draw ratio is less than8.
 11. A process according to claim 1, wherein the thermoplastic polymeris a crystallizable polymer.
 12. A process according to claim 11,wherein the thermoplastic polymer is a polyester.
 13. A processaccording to claim 11, wherein the inner hoop draw ratio is at least 2.14. A process according to claim 13, wherein the inner hoop draw ratiois at least
 3. 15. A process according to claim 11, wherein the axialdraw ratio is at least
 2. 16. A process according to claim 11, whereinthe thermoplastic polymer is poly(vinyl chloride).
 17. A processaccording to claim 16, wherein the inner hoop draw ratio is at least1.2.
 18. A process according to claim 16, wherein the inner hoop drawratio is at least 1.5.
 19. A process according to claim 16, wherein theratio of the axial draw ratio to the inner hoop draw ratio is in therange 0.5 to 1.0.
 20. A process according to claim 1, wherein theworkpiece has a nose portion and the tension transmitting mechanismcomprises a hauloff comprising a pair of serrated jaws in which the noseportion is gripped; a tension transmitting element having two endswherein one end is attached to the jaws and the other end is attached toa winch or a loading station to which a turning motion or mass may beapplied to apply a draw tension to the workpiece.
 21. A processaccording to claim 20, wherein the tension transmitting element isselected from the group consisting of a high tensile cable, a chain, arack and pinion mechanism, a screw mechanism and a hydraulicallyoperated draw mechanism.
 22. A process according to claim 1, wherein thetension transmitting mechanism comprises a pair of continuouscontra-rotating friction belts.
 23. A process according to claim 1,wherein the draw tension is from 1.1 kN up to but not including thefracture stress point of the workpiece.
 24. A process according to claim1, wherein the thermoplastic polymer is heated to within 60° C. of itsmelting point prior to deformation.
 25. A process according to claim 1,wherein the thermoplastic polymer has a weight average molecular weightof from 50,000 to 150,000 and is heated to 70° C. to 100° C. prior todeformation.
 26. A process according to claim 1, wherein thethermoplastic polymer has a weight average molecular weight of above300,000 and is heated to 70° C. to 120° C. prior to deformation.
 27. Aprocess according to claim 1, wherein the thermoplastic polymer isselected from linear homo-polymers and linear copolymers ofpolypropylene has a weight average molecular weight of from 150,000 to800,000 and is heated to 20° C. to 170° C. prior to deformation.
 28. Aprocess according to claim 27, wherein the thermoplastic polymer isheated to 90° C. to 130° C.
 29. A process according to claim 1, whereinthe thermoplastic polymer is a polyester and it is heated to 55° C. to100° C. prior to deformation.
 30. A process according to claim 1,wherein the thermoplastic polymer is selected from homopolyoxymethyleneand copolyoxymethylene and is heated to 80° C. to 170° C. prior todeformation.
 31. A process according to claim 30, wherein thethermoplastic polymer is heated to 150° C. to 170° C.
 32. A processaccording to claim 1, wherein the thermoplastic polymer is a vinylidenefluoride polymer and is heated to 80° C. to 165° C. prior todeformation.
 33. A process according to claim 1, wherein thethermoplastic polymer is polyvinylchloride and it is heated to about100° C. prior to deformation.
 34. A process according to claim 27,wherein the thermoplastic polymer is a polypropylene and is heated toabout 135° C. or about 155° C. prior to deformation.
 35. A processaccording to claim 1, wherein the thermoplastic polymer is a highdensity polyethylene and is heated to about 115° C. prior todeformation.
 36. A process according to claim 1, wherein said hollowworkpiece has a wall thickness of 4 mm-20.5 mm.
 37. A process accordingto claim 36, wherein said wall thickness is 5 mm.